1. Field of the Invention
The present invention relates to a speaker including a voice coil driven by a single magnet and, in particular, to a speaker including a voice coil driven by a single magnet and providing the magnetic force distribution and performance identical to that of a speaker using two magnets.
2. Description of Related Art
In general, a speaker includes a frame, a diaphragm supported by the frame, and a magnetic driving unit. The magnetic driving unit includes a magnet attached to the frame, a magnetic yoke opposed to the magnet, and a coil (voice coil) which is disposed in a gap between the magnet and the opposing magnetic yoke and which moves along with the diaphragm.
In the magnetic driving unit, the coil is driven by an electromagnetic force induced by a magnetic flux emanating from the magnet to the magnetic yoke over the gap and the electrical current flowing in the coil located in the gap. Therefore, the diaphragm vibrates along with the coil to produce sound.
The strength of a magnetic field provided to the magnetic yoke by the magnetized surface of the magnet exhibits a peak at the center of the magnetized surface in the moving direction of the coil. In the vicinity of the center, as the distance from the center increases, the magnetic field strength gradually decreases. Accordingly, when the magnet opposing the gap is single and when the position where the moving coil faces the magnetized surface changes, the magnetic field through the coil changes in strength. Therefore, it is difficult to maintain the linearity of a driving force applied to the coil, which is a problem.
To address this problem, Japanese Patent No. 2917578 and Japanese Unexamined Patent Application Publication No. 8-140191 disclose a technology in which two magnets are provided with a space therebetween in the moving direction of a coil, and the coil is disposed in a gap between a yoke and the magnets such that the coil can face each magnetized surface of the two magnets. Since a single coil faces the two magnets, each of which has a peak of magnetic field strength, the large change in the strength of the magnetic field through the coil can be reduced when the coil moves. Thus, the linearity can be easily maintained. If the linearity of a driving force applied to the coil is maintained, the occurrence of sound distortion at high power output can be prevented.
The speaker disclosed in the above-described two publications uses two independent magnets so as to easily ensure the linearity of a driving force. However, the use of two magnets causes the following problems:    (1) The assembly of the speaker is difficult.    (2) The performance is not always the same among assembled speakers.    (3) The manufacturing cost increases.
As for the difficulty of assembly, the distance between the two magnets needs to be precisely determined in accordance with the length of the coil in the moving direction to ensure the linearity of a driving force applied to the coil. Accordingly, a tool such as a jig is required at assembly time in order to determine the positions of the two magnets. Furthermore, a complicated operation in which the two magnets are secured while determining their positions is required.
As for the uneven performance, it is difficult to manufacture speakers having two magnets with the same magnetic field strength and distribution. Thus, the magnetic field strengths of the two magnets are sometimes different. In this case, the linearity of a driving force applied to a coil disadvantageously differs from speaker to speaker.
As for the manufacturing cost, separately manufacturing two magnets increases the cost of a magnetic driving unit including the two magnets.